Magnetic array method to aid relocation of subsea wells

ABSTRACT

THE SPECIFICATION DISCLOSES THE METHOD OF DISPOSITION AN ARRAY OF PERMANENT MAGNETS ON THE SEA FLOOR SURROUNDING A SUBSEA WELL TO BE ABANDONED. THE PERMANENT MAGNETS GENERATE A UNIQUE MAGNETIC FIELD TO AID IN RELOCATING THE SUBSEA WELL USING MAGNETIC DETECTION METHODS.

Feb. 23, 1971 F. E. BLouNT ErAL 3,566,257

MAGNETIC ARRAY METHOD T0 AVID RELOCATION OF' SUBSEA WELLS Filed sept. so, 196s Fig. I

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'-DEAST -\MAGNET SOUTH JNVENTORS FLOYD E. BLOUNT WALLACE B. ALLEN www ATTORNEY United States Patent O 3,566,257 MAGNETIC ARRAY METHOD TO AID RELOCATION OF SUBSEA WELLS Floyd E. Blount and Wallace B. Allen, Dallas, Tex., as-

signors to Mobil Oil Corporation, a corporation of New York Filed Sept. 30, 1968, Ser. No. 763,713 Int. Cl. G01r 33/00 U.S. Cl. 324-34 5 Claims ABSTRACT OF THE DISCLOSURE The specification discloses the method of depositing an array of permanent magnets on the sea floor surrounding a subsea well to be abandoned. The permanent magnets generate a unique magnetic field to aid in relocating the subsea well using magnetic detection methods.

BACKGROUND OF THE INVENTION Sometimes it is desirable to temporarily abandon subsea oil and gas `wells and to relocate them at a later date. For example, when a gas well is discovered it may be advantageous to cap the well and return to produce the gas at a more favorable time. The easiest technique for relocating an abandoned subsea well requires leaving some equipment, such as a buoy, projecting above the water surface to mark the location of a well. For various reasons, temporarily abandoned subsea wellheads may not have any equipment projecting above the water surface to permit easy relocation. Governmental regulations may prescribe that a subsea wellhead not project above the sea floor to prevent interference with shipping or fishing.

The state of the art for relocating abandoned subsea wellheads includes (l) searching by radio ranging techniques (sonar), (2) dragging a cable across the sea floor to intercept a projecting wellhead, (3) visual sighting by divers, and (4) attaching an acoustic pulsing device or pinger to the wellhead and listening for the characteristic pulse pattern of the pinger. If an abandoned subsea wellhead is flush with or below the sea floor, it may be very diflicult even for a diver in the immediate vicinity of the wellhead to spot itparticularly if the water is murky. Sand on the sea floor may shift so that a subsea wellhead previously ilush with the sea floor becomes buried. The pulses emitted by acoustic devices cannot be detected successfully at large distances when the well is buried. Furthermore, acoustic devices or any devices requiring an active element are not inherently failproof because of the problems of corrosion, loss of battery life, and malfunctions of circuitry and instruments. Underwater type batteries rarely have a useful life or more than two years.

A truly failproof technique to aid in relocating a temporarily abandoned well is by magnetic methods. The magnetic technique involves searching lwith a magnetometer for the magnetic anomaly created by ferromagnetic material in the vicinity of a subsea well. The magnetic techniques may be used as alternative or secondary methods after the primary methods, such as the sonic devices, fail. Usually, radio or celestial navigational aids can pinpoint the location of the well within a few miles. It remains for other techniques to 'be employed to pinpoint the Well within a defined search area.

In the patent application of Bobbie I. Patton, Ser. No. 762,958, filed Sept. 26, 1968, there is described a method to aid relocation of a subsea Well by inducing remanent magnetism in the casing of the well before it is put in place. Also, in our patent application Ser. No. 762,469, filed Sept. 25, 1968, there is described a mechanical ice assembly `for attachment to a subsea wellhead to enhance the magnetic anomaly in the vicinity of the well.

SUMMARY OF THE INVENTION The present invention provides a method to aid relocation of a temporarily abandoned subsea well. The method comprises the step of depositing permanent magnets on the sea floor in an array surrounding the well to be abandoned. The array of permanent magnets generates a unique and identifiable magnetic field which will aid in relocating the exact position of a well at a later date. The magnetic array may comprise a series of concentric rectangles or a spiral whose center is at the well.

When a well is to be abandoned a vessel may sail away from the well along the magnetic array pattern and drop the permanent magnets overboard at spaced intervals. The permanent magnets are preferably comprised of a ceramic material so that they will not be subject to environmental deterioration. Each of the permanent magnets may be weighted to improve the accuracy of deposition on the sea floor.

The location where each magnet is deposited may be recorded according to celestial or radio navigational aids so that at a later date when a vessel returns with a magnetic detecting instrument the position of magnetic anomalies generated by each magnet in the array may be correlated with the location according to the navigational aid.

In the accompanying drawing, FIG. 1 is a schematic diagram of the method of depositing magnets in an array of concentric rectangles and FIG. 2 illustrates the same method `for a spiral array.

Referring to FIG. 1, a subsea wellhead 10 is illustrated surrounded by an array of permanent magnets deposited at locations marked by Xs. The square-shaped magnetic array pattern of FIG. 1 is chosen as an example to illustrate the present invention.

In depositing the magnets it is convenient for a vessel 12 to begin at the wellhead 10 which serves as a reference point. By moving in a spiral-like pattern along the path indicated by the dashed line, the vessel 12 may efficiently deposit permanent magnets on the sea floor along a series of concentric squares. More specifically, vessel 12 may begin at the location of wellhead 10 and sail in an easterly direction and deposit a magnet at location 14. Then it may make a right angle turn and sail southerly to the location for dropping a magnet at 16. Then vessel 12 will sail westerly along line 17 and drop magnets at locations 18 and 19. Vessel 12 will continue to drop magnets at the locations marked by Xs as shown in FIG. 1.

At each location where a magnet is dropped, a record is made of the location according to navigational aids. Thus, the position where each magnet is deposited may be plotted on a chart or map.

The number of magnets in each row in FIG. 1 may be different so that it will be easier for a returning vessel to distingiush which row of permanent magnets is being detected. Also, the intervals between magnet locations may be variable between rows or within each row, thereby providing further unique identification for the rows of permanent magnets within the entire pattern.

The magnetic array pattern of FIG. l includes permanent magnets along a radius extending outward from wellhead 10 so that a returning vessel may sail directly to the wellhead 10 once sufficient information is known about the entire array pattern. For example, magnets may be deposited along the north, east, south, and west lines as shown in FIG. 1 and along the diagonals 22 and 24 of the square array pattern.

Assume that a vessel returning with magnetic detector instrumentation is able to follow along the outside row 26 in a westerly direction. If the number of magnets detected in row 26 are counted and compared with the previously recorded number of magnets deposited, it may be 'known when the returning vessel reaches the west end of row 26. Thereupon the returning vessel may sail directly toward well along diagonals 24 following the course indicated by the magnetic anomalies.

The retrieval vessel may use any of various known types of search techniques and magnetic detection instruments to relocate wellhead 110 by sensing the unique magnetic anomaly generated by the magnetic array. Commercial magnetometer services are oteredr by various contractors such as Oceanics, Inc., Morgan City, La.

The magnetic detection instruments carried by the retrieval vessel is preferably towed at depth in the water and slightly above the bottom whereby maximum sensitivity is afforded. The towed magnetometer instrument may be provided with variable depth control means to permit maintaining the instrument slightly above the sea oor. For example, the magnetometer instrument may be enclosed in a chamber whose buoyancy is adjustable by remote control from the surface vessel so that the depth of the instrument may be varied. Alternatively, the towed magnetometer may include diving planes to permit variable depth control.

To shorten search time, several laterally spaced magnetometers may be towed by the search vessel, each magnetometer being provided with an individual readout by instrumentation on the surface vessel.

yOf course, there are many patterns of magnets that could be employed, the pattern of FIG. l being only one example. Another example of a magnetic array pattern is shown in iFIG. 2 in the form of a spiral whose center is at the well-head 10'. In laying out the magnetic array of FIG. 2, the vessel 12 may begin at wellhead 10 and sail outwardly in circles of ever-increasing diameter, depositing permanent magnets along the way as indicated by the locations marked with Xs. The interval between the magnetic drop points may be constant or variable according to a desired plan. For example, as shown in FIG. 2 the interval between magnet drop points may increase with increase in distance from the wel'lhead 10.

The array of permanent magnets surrounding a subsea wellhead may extend over an area as large as may be desired, but preferably within the tolerance of the position of a subsea well according to conventional navigational aids. For example, if a wellhead can be located within a tolerance of ten miles according to radio navigational aids, the boundaries of the magnetic array preferably is encompassed by a circle with a radius of ten miles. Thus, as soon as the retrieval vessel returns to within the area designated by the navigational aid, it may begin to detect the magnetic anomalies created by the deposited magnets.

The invention claimed is:

:1. A method to aid relocation of a temporarily abandoned subsea well comprising the step of depositing a plurality of permanent magnets on the sea Ifloor, each of said magnets being spaced at a predetermined distance from said Well and arranged in a deiined relationship with respect to each other, some of said magnets being positioned relatively near said well while others of said magnets are spaced at greater and greater distances from said well whereby said magnets form. an array covering a relatively large area on the sea -oor which generates an identiliable magnetic eld which can be detected by a surface vessel passing over said area covered by said array.

'2. A method as defined by claim 1 in which the array defines a series of concentric rectangles.

3. A method as defined by claim I1 in which the array defines a spiral whose center is at the well to be abandoned.

4. A method as dened by claim 1 in which the permanent magnets are comprised of ceramic material.

5. A method as defined by claim 1 further including the step of recording the location where each permanent magnet is deposited by means of a navigational aid.

References Cited UNITED STATES PATENTS 3,027,951 4/1962 Knapp et al. 175-10 2,527,681 10/1950 Lewis, Jr. et al. 33'5--305 OTHER REFERENCES Wadsworth, NJ.: A Proton lFree-Precession Magnetometer for Locating -Buried Magnets, Journal of Scientic Inst., 1967, vol. 44, p. 552.

RUDOLPH V. ROLINfEC, Primary Examiner IR. I. CORCOtRAN, Assistant Examiner 

